1. Field of the Invention
This invention relates to a dual filtering system by which fluids may be clarified by separating constituents suspended in the fluids from a filtrate. More particularly, this invention relates to a dual filtering system by which a liquid or gas may be cleansed of suspended particles by passing the liquid or gas, under a pressure differential, serially through a sand filter medium and a diatomaceous earth filter medium which transmit the fluid and capture the particles.
2. Description of Prior Art
Filtering systems for filtering fluids, such as water for use in a swimming pool or the like, are well known and include those types in which sand, diatomaceous earth (DAE), flexible foams such as polyurethanes and polyamides, or pleated polyester cartridge filter mediums are employed.
In sand filtration systems, water from the pool is pumped through a bed of sand and collected by an underdrain, into which the water, but not the sand, can pass. The upstream surface of the sand bed often becomes clogged by large particles and an impermeable layer forms on the upstream surface. The resultant increased pressure drop across this impermeable layer decreases the filtration rate until filtration almost stops requiring the cleaning or replacement of the sand before it has been permeated by the smaller particles.
In DAE filtration systems, a pressure differential in the fluid being filtered is maintained at opposite sides of each DAE filtering element. The DAE filter elements may be in the form of porous or finely perforated, vertically aligned, curved rectangular plates or cylindrical tubes. In the operation of such filter elements, fluid containing DAE particles in suspension is forced through the porous filter plates or tubes and DAE particles accumulate on the surface of the plates or tubes so as to reduce the size of the interstices or perforations on the plate or tube wall and thus enhance the filtering operation and ensure the removal from the fluid of particles suspended therein. It is necessary periodically to remove accumulated DAE particles from the filter plates or tubes to prevent clogging thereof.
The plates comprise one or more hollow, rectangular inner support elements, which are perforated in a grid-like pattern. The elements are radially equidistantly disposed about an axis parallel to the longitudinal axis of the rectangular plate, and have one or more longitudinal support ribs. The inner support element is encased within an outer, fine mesh polyester cloth which passes water but retains the DAE particles. In operation, when the filtering cycle is stopped, the accumulated DAE layer falls off the outer surface of the plates. However, it has been found that accumulated DAE particles tend not to fall off at the corners and sides of such outer surfaces.
The tubes may comprise one or more layers of fine mesh wire screen mounted on rigid supports such as wire frames and often the DAE powder or the like accumulates and builds up in the spaces between the tubes. The removal of the accumulated layer of powder or "sludge", is usually effected by a so-called backwash operation wherein liquid is forced through the tubes in the direction opposite to that in which the liquid flowed during the filtering operation. It has been extremely difficult and sometimes impossible, to completely remove the accumulated layer of powder from the tubes, and too often the sludge builds up or "bridges" in the spaces between the tubes which seriously interferes with the removal of sludge from the filter. Sometimes, means has been provided for mechanically removing the sludge from the filter by vigorously shaking or tapping the tubes, but this operation not only fails to accomplish a satisfactory removal of the layer of diatomaceous earth, but also may result in damage to the tubes, for example, cracking or puncturing thereof.
U.S. Pat. Nos. 3,100,190 and 3,642,141, both to Hobson, Jr. describe tube sheet assemblies wherein a plurality of flexible tubes are placed within corresponding openings in a cover plate and their top ends affixed to the cover plate. The tubes are vertically aligned and densely packed in this arrangement. Each tube has a sock-like outer side wall which is woven to permit liquid to pass through openings therein and which is flexible. Each tube also has an inner porous frame which is of smaller diameter than the diameter of the side wall and also may be inherently resilient along its longitudinal axis to permit the outer tube to be bent during a filtering operation. Because of this construction, the tubes will change in length and in diameter in response to changes in the pressure of the backwash liquid and thereby allegedly will loosen an accumulated outer layer of DAE or similar "filter aid" from the wall of the tube and cause the dislodged layer to fall away from the tube by gravity. Removal of the accumulated layer of "filter aid" or DAE from the tubes is further facilitated by so-called "bumping" of the tubes, i.e., causing rapid up and down movement of the tubes which in turn causes the tubes to sway and shake until dislodged material falls off.
In practice, however, it has been found that the flexible nature of such tubes causes them to tangle and interlock with one another such that: (1) the "bumping" does not dislodge accumulated layers of "filter aid"; (2) they cannot be easily removed from the tube sheet which holds them and thus cannot be easily and separately cleaned and/or repaired; and (3) they lose about 30% of their available surface area for filtration, thereby reducing efficiency of the filtration. Moreover, the loose fitting slidable relationship between the outer side wall and the inner support frame permits opening of the woven side wall and allows DAE particles to be passed through the tubes back to the pool during the initiation of a filter cycle. Also, movement of the flexible tubes during filtering interferes with the desired accumulation of a uniform DAE layer on the tube side wall.
U.S. Pat. No. 3,954,621 to Etani et al. describes a filtration system comprising a thin pre-filter medium made of an open celled polymeric foam or a nylon woven mesh or fabric and a thick main filter medium also made of an open-celled polymeric foam, e.g., polyurethane. The pore size of the pre-filter medium is no greater than the pore size of the main filter medium.
An object of the present invention is to provide a dual filtering system providing both sand and DAE filtration. Another object of the present invention is to provide a dual filtering system in which dirt and any large particulates are removed by passing a liquid feed through a sand filter medium and minute impurities are then removed by passing the liquid filtrate through a DAE filter medium.
It is an object of this invention to provide a dual filtration system comprised of sand and DAE filter mediums in which the DAE filter medium is protected from large debris in the liquid to be filtered, thus permitting the DAE filter medium to be operated for a protracted period of time with little, if any, need to backwash the DAE filter medium.
Still another object of this invention is to provide a dual filtration system which is simple and economical to operate and maintain.
Another object of this invention is to provide a dual filtration system for swimming pools which has the ability to remove minute particles from pool water and thereby lower the cost of chemicals otherwise needed to clean the water of these particles.
An object of the present invention is to provide a dual filtering system in which DAE filter units and a sand filter are disposed within a single tank in separate zones, one located above the other in a spaced, coaxial arrangement.
Still another object is to provide a dual filtering system having two separate zones of filtering, one above the other, in which the pressure within the two zones is automatically regulated.
An object of the present invention is to provide a dual filtering system including a sand type and a DAE type filter, the DAE filter comprised of a multiplicity of vertical filter grids, each of which is removably connected to radial arms of a manifold assembly and spaced apart from another, the grids comprised of a rigid, sock like pervious filter tube tightly fitted over a rigid, perforated tubular frame, the external surfaces of the grid units capable of being coated with a layer of DAE upon application of pressure, the DAE coating becoming easily dislodged upon removal of the pressure.
Other objects and advantages of the invention will be apparent from the accompanying drawings and description, the essential features of which are set forth herein in the appended claims.